Anti-stiction bottom cavity surface for micromachined ultrasonic transducer devices

ABSTRACT

A method of forming an ultrasonic transducer device involves depositing a first layer on a substrate, depositing a second layer on the first layer, patterning the second layer at a region corresponding to a location of a transducer cavity, depositing a third layer that refills regions created by patterning the second layer, planarizing the third layer to a top surface of the second layer, removing the second layer, conformally depositing a fourth layer over the first layer and the third layer, defining the transducer cavity in a support layer formed over the fourth layer; and bonding a membrane to the support layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 16/683,652, filed Nov. 14, 2019, which is hereby incorporated by reference in its entirety.

U.S. patent application Ser. No. 16/683,652 claims the benefit under 35 U.S.C. § 119(e) of U.S. Patent Application Ser. No. 62/768,048, filed Nov. 15, 2018, which is hereby incorporated herein by reference in its entirety.

U.S. patent application Ser. No. 16/683,652 further claims the benefit under 35 U.S.C. § 119(e) of U.S. Patent Application Ser. No. 62/810,358, filed Feb. 25, 2019, which is hereby incorporated herein by reference in its entirety.

FIELD

The present disclosure relates generally to micromachined ultrasonic transducers and, more specifically, to an anti-stiction, bottom cavity surface for micromachined ultrasonic transducer cavities and transducer manufacturing techniques.

BACKGROUND

Ultrasound devices may be used to perform diagnostic imaging and/or treatment, using sound waves with frequencies that are higher than those audible to humans. When pulses of ultrasound are transmitted into tissue, sound waves are reflected off the tissue with different tissues reflecting varying degrees of sound. These reflected sound waves may then be recorded and displayed as an ultrasound image to the operator. The strength (amplitude) of the sound signal and the time it takes for the wave to travel through the body provide information used to produce the ultrasound images.

Some ultrasound imaging devices may be fabricated using micromachined ultrasonic transducers, including a flexible membrane suspended above a substrate. A cavity is located between part of the substrate and the membrane, such that the combination of the substrate, cavity and membrane form a variable capacitor. When actuated by an appropriate electrical signal, the membrane generates an ultrasound signal by vibration. In response to receiving an ultrasound signal, the membrane is caused to vibrate and, as a result, generates an output electrical signal.

SUMMARY

In one aspect, a method of forming an ultrasonic transducer devices involves depositing a first layer on a substrate; depositing a second layer on the first layer; patterning the second layer at a region corresponding to a location of a transducer cavity; depositing a third layer that refills regions created by patterning the second layer; planarizing the third layer to a top surface of the second layer; removing the second layer; conformally depositing a fourth layer over the first layer and the third layer; defining the transducer cavity in a support layer formed over the fourth layer; and bonding a membrane to the support layer.

In one aspect, a method of forming an ultrasonic transducer device involves depositing a first layer on a substrate; patterning the first layer at a region corresponding to a location of a transducer cavity; conformally depositing an additional layer of a same type as the first layer on the first layer and exposed portions of the substrate; conformally depositing a second layer on the additional layer of the same type as the first layer; conformally depositing a third layer on the second layer; conformally depositing a fourth layer on the third layer; planarizing the fourth layer to a top surface of the third layer; depositing a membrane support layer; defining the transducer cavity in the support layer, wherein exposed portions of the third layer and the fourth layer are removed; and bonding a membrane to the support layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and embodiments of the application will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same reference number in all the figures in which they appear.

FIG. 1 is a cross-sectional view of an exemplary micromachined ultrasonic transducer device.

FIG. 2 is a cross-sectional view of an exemplary micromachined ultrasonic transducer device having an anti-stiction bottom cavity surface according to an embodiment.

FIG. 3 is top view of an anti-stiction bottom cavity surface according to one embodiment.

FIG. 4 is top view of an anti-stiction bottom cavity surface according to another embodiment.

FIG. 5 is top view of an anti-stiction bottom cavity surface according to another embodiment.

FIG. 6 is top view of an anti-stiction bottom cavity surface according to still another embodiment.

FIG. 7 is an enlarged view of a portion of FIG. 5 .

FIG. 8 is a cross-sectional view of the anti-stiction bottom cavity surface of FIG. 7 , taken along the line 8-8.

FIGS. 9-1 through 9-7 are a series of cross-sectional views illustrating an exemplary process flow for forming a micromachined ultrasonic transducer device having an anti-stiction bottom cavity surface according to an embodiment.

FIG. 10 is a flow diagram describing the exemplary process flow of FIGS. 9-1 through 9-7 .

FIGS. 11-1 through 11-9 are a series of cross-sectional views illustrating an exemplary process flow for forming a micromachined ultrasonic transducer device having an anti-stiction bottom cavity surface according to another embodiment.

FIG. 12 is a flow diagram describing the exemplary process flow of FIGS. 11-1 through 11-9 .

FIG. 13 is a top view of an example ultrasonic transducer device formed that may be formed using the process flow of FIGS. 9-1 through 9-7 and FIG. 10 , or using the process flow of FIGS. 11-1 through 11-9 and FIG. 12 .

DETAILED DESCRIPTION

The techniques described herein relate to an anti-stiction, bottom cavity surface for micromachined ultrasonic transducer cavities.

One type of transducer suitable for use in ultrasound imaging devices is a micromachined ultrasonic transducer (MUT), which can be fabricated from, for example, silicon and configured to transmit and receive ultrasound energy. MUTs may include capacitive micromachined ultrasonic transducers (CMUTs) and piezoelectric micromachined ultrasonic transducers (PMUTs), both of which can offer several advantages over more conventional transducer designs such as, for example, lower manufacturing costs and fabrication times and/or increased frequency bandwidth. With respect to the CMUT device, the basic structure is a parallel plate capacitor with a rigid bottom electrode and a top electrode residing on or within a flexible membrane. Thus, a cavity is defined between the bottom and top electrodes. In some designs (such as those produced by the assignee of the present application for example), a CMUT may be directly integrated on an integrated circuit that controls the operation of the transducer. One way of manufacturing a CMUT is to bond a membrane substrate to an integrated circuit substrate, such a complementary metal oxide semiconductor (CMOS) substrate. This may be performed at temperatures sufficiently low enough to prevent damage to the devices of the integrated circuit.

Referring initially now to FIG. 1 , there is shown a cross-sectional view of an exemplary micromachined ultrasonic transducer device 100, such as a CMUT. The transducer device 100 includes a substrate, generally designated by 102, (e.g., a complementary metal oxide semiconductor (CMOS) substrate, such as silicon) having one or more layers such as for example: CMOS circuits, wiring layers, redistribution layers, insulation/passivation layers and one or more metal electrode layers 103. Such metal electrode layer(s) 103 of the substrate 102 may serve as a transducer bottom electrode. As specific substrate and transducer bottom electrode patterns are not the focus of the present disclosure, only a single example is presented in the figures. It will be appreciated, however, that the present embodiments may also be implemented in conjunction with several other transducer electrode structures including (but not limited to), for example: donut shaped electrodes (e.g., interior metal removed), multiple segment or ring electrodes, and additional metal patterns used for other purposes besides bottom electrodes (e.g., cavity getter during bonding).

Still referring to FIG. 1 , it will be seen that the substrate 102 (including bottom electrode) may have one or more insulating layers formed thereon, generally designated by stack 104. A transducer cavity 105 is defined by lithographic patterning and etching of a support layer 106 that is formed on stack 104. The support layer 106 may be an insulating layer, such as SiO₂ for example, the remaining portions of which provide a support surface to which a flexible transducer membrane 108 (e.g., highly doped silicon at a concentration of about 1×10¹⁸ atoms/cm³ to about 1×10¹⁹ atoms/cm³) is bonded.

During operation of the transducer device 100, whether in a transmit mode and/or a receive mode, the transducer membrane 108 may come into physical contact with the top of the stack 104 (i.e., the bottom surface of the cavity 105), as indicated by the dashed position of the membrane 108′ in FIG. 1 . This position may be indicative of the so-called “collapse mode” of operation as known in the art. A “collapse mode” (also used interchangeably with the term “collapsed mode”) refers to a mode of transducer operation in which at least one portion of a micromachined ultrasonic transducer membrane is mechanically fixed and at least one portion of the membrane is free to vibrate based on a changing voltage differential between the bottom electrode and the top membrane. On one hand, operating a micromachined ultrasonic transducer in collapse mode may optimize the acoustic power (output pressure) produced by the transducer. On the other hand, however, one side effect of a micromachined ultrasonic transducer operating in collapse mode may be charge retention (also known as “charge trapping”), wherein the membrane and/or the bottom cavity surface undesirably stores charge or conducts leakage current. As a result, this charge retention may in turn undesirably alter an applied voltage at which membrane collapse is induced during device operation. Another possible side effect of collapse mode for the transducer 100 of FIG. 1 may be stiction, which is associated with the amount of surface area of contact between the membrane 108 and the bottom surface of the cavity 105 wherein the two surfaces inadvertently stick to one another. Stiction generally will lead to device reliability failure, and hence should be addressed to make sure the device can perform properly through its operation lifetime.

Accordingly, FIG. 2 illustrates a cross-sectional view of an exemplary micromachined ultrasonic transducer device 200 having an anti-stiction bottom cavity surface according to an embodiment. For ease of illustration, like elements among the various figures are designated with the same reference numbers. In the embodiment of FIG. 2 , the substrate 102 (including bottom electrode) may have one or more insulating layers formed thereon, generally designated by stack 204. In contrast to the embodiment of FIG. 1 , the uppermost layer of stack 204 is characterized by having topographic features, so as to reduce the amount of direct surface contact area between the collapsed membrane 108′ and the top layer of the stack 204.

Although there are several possible topographic patterns that may be used for the stack 204, some specific examples are illustrated in FIGS. 3-6 , each of which depicts a top view of an anti-stiction bottom cavity surface according to various embodiments. As indicated by the legends in FIGS. 3-6 , the darker and lighter shades represent different elevations of a same anti-stiction material layer (as opposed to the shades representing different materials located at a same elevation). In the embodiment of FIG. 3 , the topographic (e.g., raised) regions 302 a may have a generally rectangular shape, disposed in generally concentric circular patterns from a cavity center. The topographic regions 302 a (as well as the additional darker shaded region 302 b of the stack 204 proximate the outer perimeter of the cavity) are disposed at a higher elevation than the lighter shaded regions 304. As will be described in further detail, the topmost layer of the stack 204 may include a thin film, anti-stiction layer conformally deposited over previously defined topography located in a lower layer(s) of the stack 204. In the embodiment of FIG. 4 , the topographic regions 402 a may be generally circular and/or ring shaped, and optionally segmented. Again, as is with the case of the FIG. 3 embodiment, topographic regions 402 a (as well as the additional darker shaded region 402 b of the stack 204 proximate the outer perimeter of the cavity) are at a higher elevation than lighter shaded regions 404.

Still another embodiment for the topographic patterns that may be used for the stack 204 is illustrated in FIG. 5 . In this embodiment, the topographic regions 502 a may be defined by an array of circular posts, generally arranged in rows and columns. As compared to an entire cavity region, the lighter shaded, lower elevation region 504 may have a relatively small area with to that of the remaining outer regions 502 b of the stack 204 at the higher elevation. This embodiment may be advantageous, for example, with transducer operating modes where only a relatively small area of the transducer membrane comes into contact with the bottom cavity surface during collapse mode. On the other hand, FIG. 6 is an alternative embodiment for the stack 201, similar to FIG. 5 , but with a greater number of topographic regions 502 a (posts). The FIG. 6 embodiment may be advantageous, for example, with transducer operating modes where a relatively large area of the transducer membrane comes into contact with the bottom cavity surface during collapse mode.

An enlarged view of the dashed square region of FIG. 5 is shown in FIG. 7 , and FIG. 8 is a corresponding cross-sectional view of the stack 204 taken along the line 8-8 of FIG. 7 . By way of illustration only, an exemplary topographic configuration may have the topographic regions 502 a (posts) dimensioned with a first diameter, d₁, on the order of about 3 microns (μm) and spaced at a pitch of about 3 μm. Correspondingly, a second diameter, d₂ of the lower elevation region 504 may be on the order of about 40 μm. Again, for the FIG. 5 embodiment, d₂ may be relatively small as compared to an exemplary diameter of the entire transducer cavity (e.g., about 200 μm). Alternatively, for an embodiment such as in FIG. 6 , d₂ may be relatively larger (e.g., about 140 μm) as compared to the diameter of the entire transducer cavity (e.g., about 200 μm). It will be appreciated that these dimensions are provided for the sake of illustration only, and it is contemplated that other geometric dimensions and configurations are also possible.

Referring generally now to FIGS. 9-1 through 9-7 , there is shown a series of cross-sectional views illustrating an exemplary process for forming a micromachined ultrasonic transducer device having an anti-stiction bottom cavity surface according to an embodiment. In addition, FIG. 10 is an accompanying flow diagram describing the exemplary processing cross-sectional views of FIGS. 9-1 through 9-7 . It will be appreciated that the exemplary process flow may be used to form a device such as shown in any of FIGS. 2-8 , as well as other topographic structures not specifically depicted in the previously described drawings. As such, it should be understood that the topographic features depicted in the process flow of FIGS. 9-1 through 9-7 are for illustrative purposes only and should not otherwise be construed in any limiting sense.

As shown in FIG. 9-1 and indicated in block 1002 of FIG. 10 , a first type layer 902 and a second type sacrificial layer 904 are deposited on a transducer lower electrode layer. More specifically, a first type layer 902 is deposited on substrate 102, and a second type sacrificial layer 904 is deposited on the first type layer 902. Again, because specific transducer bottom electrode patterns are not the focus of the present disclosure, such metal electrode layer(s) 103 should be understood to be generally incorporated into an upper portion of the substrate 102 as represented in the figures. In an exemplary embodiment, the first type layer 902 may be a thin film layer of SiO₂, formed at a thickness of about 10-30 nanometers (nm), and the second type sacrificial layer 904 may be a thin film layer of SiN, formed at a thickness of about 30-70 nm. As shown in FIG. 9-2 and indicated in block 1004 of FIG. 10 , the second type sacrificial layer 904 is patterned (e.g., by photolithography and etching) at a region corresponding to the location of the transducer cavity.

Following patterning of the second type sacrificial layer 904, a third type layer 906 may be deposited as shown in FIG. 9-3 and indicated in block 1006 of FIG. 10 . The third type layer 906 may be the same type material as the first type layer 902 (e.g., SiO₂) and formed at a thickness (e.g., 400-700 nm) that is sufficient to refill the regions created by patterning and removal of portions of the second type sacrificial layer 904. Then, as shown in FIG. 9-4 and indicated in block 1008 of FIG. 10 , the third type layer 906 is planarized to the level of the top surface of the patterned second type sacrificial layer 904, such as by chemical mechanical polishing (CMP) for example. The second type sacrificial layer 904 may then be selectively removed (block 1010 of FIG. 10 ), such as by etching for example, to result in the structure shown in FIG. 9-5 .

As shown in FIG. 9-6 and indicated in block 1012 of FIG. 10 , the insulating stack 204 is defined by conformal deposition of a fourth type thin film layer 1008 over the topography of layers 902 and 906, such as by atomic layer deposition (ALD). Since the exemplary embodiment may use a same type material for layer 902 and 906 (e.g., SiO₂), the combination of layers 902/906 are depicted in subsequent figures as a single layer 910 for ease of illustration. The fourth type thin film layer 908 is selected to be an anti-stiction material, such as aluminum oxide formed at a thickness of about 20-40 nm. By conformally depositing the thin film aluminum oxide layer 908 over the topography defined by layer 910 (i.e., layers 902 and 906), a relatively uniform, continuous thin film of anti-stiction material is produced (as opposed to, for example, etching through a thin, planar layer of anti-stiction material which may result in a non-smooth, discontinuous top layer of stack 204). This in turn may be beneficial for a subsequent bonding process by, for example, providing a superior bonding interface with minimal process induced defects. Once the thin film layer 908 is formed in FIG. 9-6 , additional transducer processing may continue as known in the art, such as shown in FIG. 9-7 and indicated in block 1014 of FIG. 10 . This may include, for example, defining a transducer cavity in an insulating (support) layer 106 and bonding the flexible membrane 108 to patterned support portions of layer 106 as previously described. It should further be appreciated at this point that although the illustrated embodiments depict a single cavity, any suitable number of cavities and corresponding electrode structures may be formed (e.g., hundreds, thousands, tens of thousands, etc.)

Referring generally now to FIGS. 11-1 through 11-9 , there is shown a series of cross-sectional views illustrating an exemplary process for forming a micromachined ultrasonic transducer device having an anti-stiction bottom cavity surface according to another embodiment. In addition, FIG. 12 is an accompanying flow diagram describing the exemplary processing cross-sectional views of 11-1 through 11-9. It will be appreciated that the exemplary process flow may be used to form a device such as shown in any of FIGS. 2-8 , as well as other topographic structures not specifically depicted in the previously described drawings. As such, it should again be understood that the topographic features depicted in the process flow of FIGS. 11-1 through 11-9 are for illustrative purposes only and should not otherwise be construed in any limiting sense.

As shown in FIG. 11-1 and indicated in block 1202 of FIG. 12 , a first type layer 1102 is deposited on substrate 102 (e.g., a thin film layer of SiO₂, formed at a thickness of about 10-30 nm) and patterned (e.g., by photolithography and etching) at a region corresponding to the location of the transducer cavity. Again, because specific transducer bottom electrode patterns are not the focus of the present disclosure, such metal electrode layer(s) 103 should be understood to be generally incorporated into an upper portion of the substrate 102 as represented in the figures. As shown in FIG. 11-2 and indicated in block 1204 of FIG. 12 , an additional layer 1104 of the first type material (e.g., a thin film layer of SiO₂, formed at a thickness of about 10-30 nm) is conformally deposited over the patterned layer 1102 and the exposed portions of the metal electrode layer(s) 103. The resulting layer 1106 (shown as a single layer in subsequent figures) has a desired bottom cavity layer topography, such as for example one corresponding to any of the patterns discussed above or other patterns not explicitly described above. It will be noted that an alternative way to form the intermediate structure shown in FIG. 11-2 could be to form a thicker layer of first type material (e.g., a thin film layer of SiO₂, formed at a thickness of about 20-60 nm), followed by a timed etch (i.e., one that does not go completely through the SiO₂ material down to the metal electrode layer(s) 103). However, the latter method as shown in the FIGS. 11-1 and 11-2 may provide a more reliable way to control topographic features such as step heights of subsequently formed layers.

Following the formation of the first type material topographical layer 1106 and as indicated in block 1206 of FIG. 12 , a second type material layer 1108 is conformally deposited over layer 1106 as shown in FIG. 11-3 , followed by a third type material layer 1110 conformally deposited over layer 1108 as shown in FIG. 11-4 . The second type material layer 1108 may be an anti-stiction material, such as aluminum oxide formed by ALD at a thickness of about 20-40 nm. The third type material layer 1110 is selected to as to act as a CMP stop layer, and may be for example a SiN layer formed at an initial thickness of about 20-50 nm.

As shown in FIG. 11-5 and indicated in block 1208 of FIG. 12 , a fourth type layer 1112 (e.g., thin film SiO₂) is conformally deposited over the third type material/CMP stop layer 1110 at a thickness (e.g., about 400-700 nm), and may act as a CMP buffer layer. Then, in FIG. 11-6 and as indicated in block 1210 of FIG. 12 , the structure is planarized so as to substantially remove the CMP buffer layer 1112, stopping on the CMP stop layer 1110, the thickness of which may be reduced as a result of the CMP operation. Some portions of the CMP buffer layer 1112 might still remain over the lower topographic regions of the CMP stop layer 1110, as also shown in FIG. 11-6 . An advantage of the above described combination of the CMP buffer layer 1112 and CMP stop layer 1110 may be to provide an anti-stiction surface 1108 integrated into a transducer device while the processing operations are still compatible with a CMUT planarization process, thereby providing a superior bonding interface with minimal process induced defects.

Following planarization, additional transducer processing operations may continue as known in the art. As shown in FIG. 11-7 and indicated in block 1212 of FIG. 12 , the membrane support layer 106 (e.g., SiO₂) is formed. Then, as shown in FIG. 11-8 and indicated in block 1214 of FIG. 12 , the membrane support layer 106 is lithographically patterned and etched to define the transducer cavity 105, where the exposed portions of the membrane support layer 106, the CMP polish stop layer 1110 and remaining CMP buffer layer 1112 are removed so as to expose the topographic anti-stiction layer 1108. The flexible membrane 108 may then be bonded to the remaining portions of the membrane support layer 106 as shown in FIG. 11-9 and indicated in block 1216 of FIG. 12 . Again, it should further be appreciated at this point that although the illustrated embodiments depict a single cavity, any suitable number of cavities and corresponding electrode structures may be formed (e.g., hundreds, thousands, tens of thousands, etc.)

FIG. 13 illustrates a top view of an example ultrasonic transducer device 1300 formed using any of the exemplary process flow embodiments described herein. As illustrated, the transducer device includes an array of individual transducers 100, such as those described above. The specific number of transducers 100 shown in FIG. 13 should not be construed in any limiting sense, and may include any number suitable for a desired imaging application, which may be for example on the order of tens, hundreds, thousands, tens of thousands or more. FIG. 13 further illustrates an example location of metal 1302 that may distribute an electrical signal to the membranes (upper electrodes) of the transducers 100.

It should be appreciated that although the exemplary geometric structure of this portion of the ultrasonic transducer 100 is generally circular in shape, other configurations are also contemplated such as for example, rectangular, hexagonal, octagonal, and other multi-sides shapes, etc.

The above-described embodiments can be implemented in any of numerous ways. For example, the embodiments may be implemented using hardware, software or a combination thereof. When implemented in software, the software code can be executed on any suitable processor (e.g., a microprocessor) or collection of processors, whether provided in a single computing device or distributed among multiple computing devices. It should be appreciated that any component or collection of components that perform the functions described above can be generically considered as one or more controllers that control the above-discussed functions. The one or more controllers can be implemented in numerous ways, such as with dedicated hardware, or with general purpose hardware (e.g., one or more processors) that is programmed using microcode or software to perform the functions recited above.

Various aspects of the present invention may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments.

Also, some aspects of the technology may be embodied as a method, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively. 

What is claimed is:
 1. A method of forming an ultrasonic transducer device, the method comprising: depositing a first layer on a substrate; depositing a second layer on the first layer; patterning the second layer at a region corresponding to a location of a transducer cavity; depositing a third layer that refills regions created by patterning the second layer; planarizing the third layer to a top surface of the second layer; removing the second layer; conformally depositing a fourth layer over the first layer and the third layer; defining the transducer cavity in a support layer formed over the fourth layer; and bonding a membrane to the support layer.
 2. The method of claim 1, wherein depositing the first layer on the substrate comprises depositing the first layer on a transducer lower electrode layer incorporated into an upper portion of the substrate.
 3. The method of claim 1, wherein the first layer comprises silicon oxide.
 4. The method of claim 1, wherein the first layer is formed at a thickness of about 10-30 nanometers.
 5. The method of claim 1, wherein the second layer comprises silicon nitride.
 6. The method of claim 1, wherein the second layer is formed at a thickness of about 30-70 nanometers.
 7. The method of claim 1, wherein the third layer is a same material as the first layer.
 8. The method of claim 1, wherein the third layer comprises silicon oxide.
 9. The method of claim 1, wherein the third layer is formed at a thickness of about 400-700 nanometers.
 10. The method of claim 1, wherein planarizing the third layer comprises using chemical mechanical polishing.
 11. A method of forming an ultrasonic transducer device, the method comprising: depositing a first layer on a substrate; patterning the first layer at a region corresponding to a location of a transducer cavity; conformally depositing an additional layer of a same type as the first layer on the first layer and exposed portions of the substrate; conformally depositing a second layer on the additional layer of the same type as the first layer; conformally depositing a third layer on the second layer; conformally depositing a fourth layer on the third layer; planarizing the fourth layer to a top surface of the third layer; depositing a membrane support layer; defining the transducer cavity in the support layer, wherein exposed portions of the third layer and the fourth layer are removed; bonding a membrane to the support layer.
 12. The method of claim 11, wherein depositing the first layer on the substrate comprises depositing the first layer on a transducer lower electrode layer incorporated into an upper portion of the substrate.
 13. The method of claim 11, wherein the first layer comprises silicon oxide.
 14. The method of claim 11, wherein the first layer is formed at a thickness of about 10-30 nanometers.
 15. The method of claim 11, wherein the additional layer of the same type as the first layer is formed at a thickness of about 10-30 nanometers.
 16. The method of claim 11, wherein the additional layer of the same type as the first layer is formed at a first thickness and then etched down to a second thickness.
 17. The method of claim 11, wherein the third layer comprises silicon nitride.
 18. The method of claim 11, wherein the third layer is formed at a thickness of about 20-50 nanometers.
 19. The method of claim 11, wherein the fourth layer comprises silicon oxide.
 20. The method of claim 11, wherein the fourth layer is formed at a thickness of about 400-700 nanometers. 